Deep web describes web sites, which offer dynamically generated web pages in response to an end-user submitted query. For example, US Postal Service (USPS) offers shipment tracking on the web by allowing end-users to submit tracking numbers. The tracking number is processed by looking up its entry in the USPS database for the time and location of the referenced shipment. Similarly, at the Hertz web site, an end-user can submit a query for a specific date, location and car model to check the availability of vehicles for rent. This information is also composed of query results of the Hertz database. Both USPS and Hertz offer deep web services, in addition to statically linked web pages for generic informational content.
It has been observed that deep web services are increasingly being offered to allow business activities and commerce transactions on the web. A common theme is that they use web forms for users to fill and submit formatted queries. As in the previous examples, USPS asks for tracking number while Hertz asks for date, location and vehicle category. Unlike static web pages, these web forms make it very difficult for search engine robots to crawl the backend databases. Since search engines do not find deep web content, deep web sometimes is referred to as the hidden web.
As deep web services proliferate, it becomes critical to understand, organize and search them.